Packer seals are subjected to differential pressures when set. The differential pressure creates shear forces in the seal material that can be significant enough to cause extrusion of the seal material in the axial direction. To address this issue extrusion rings have been used. These rings are disposed on opposed ends of the seal assembly for a packer and are typically overlapping rings that deform to span an annular gap as the seal assembly is either mechanically compressed or swells in the presence of a predetermined fluid that is found at the packer location. Generally the backup rings are deformed plastically to span the annular gap with the seal. In most applications the packer or bridge plug is not design to be retrieved so issues with release of the backup rings do not arise. However, if the packer is to be retrieved and the mechanically compressing force that holds the sealing element in the set position is released, the backup rings continue in their set position. When an attempt is made to move the now unset packer there is a possibility that the backup rings could bend and deform in a manner that would either require destroying the backup rings or facing a situation where the released packer is jammed with contorted backup rings forcing a release from the packer and a subsequent very expensive milling operation.
Various designs for extrusion barriers or backup rings have been tried in the past. In some applications extending members act as debris barriers to protect mechanical components of the slips from jamming with debris and thus preventing a release. These devices are generally not immediately next to the seal assembly and are more often spaced a short distance from the slips and seal assembly of the packer and are on the uphole side of the packer to prevent settling debris from getting into the packer mechanical parts after a long period where the packer can be set in the hole. Extrusion barriers are used with inflatable elements as well as mechanically compressed elements.
U.S. Pat. No. 8,561,689 shows a stack of rings that are bent into position as the sealing element swells to the set position. U.S. Pat. Nos. 7,806,177 and 8,307,891 show an array of pivoting segments to act as extrusion barriers that are secured to a slip ring so that the segments can move radially out or in with the movement of the segments of the slip ring. US 2013/0306330 uses ring segments associated with the slips to in turn reshape a round profile backup ring into an oval profile for backing up the sealing element. US2013/0147121 shows a flexible ring that is retained by opposed axially movable end rings. The backup ring moves out radially when the end rings are brought together and is forcibly retracted when the end rings move axially away from each other. Finally, WO98/35130 shows the use of articulated linkages that move radially into an anti-extrusion position when the sealing element is compressed and whose motion reverses when the bridge plug is extended to reverse.
These designs have several shortcomings. Many simply plastically deform and are not functional for another set and may actually get bent or crack under stress imposed in the set position so that attempts to get such designs to release can be futile. This means that the packer may need to be drilled out. Segmented back up rings have gaps between the segments, creating an extrusion gap, especially in high pressure high temperature applications. Some of these designs are forcibly retracted which again presents the risk of cracks at stress concentration locations and a subsequent contorting of the ring structure so as to make the packer or bridge plug hard or impossible to remove. What is needed and provided by the present invention is an expandable continuous extrusion barrier that acts as a spring in the manner that it has an open coiled structure for running in and when set the gaps between the coils close and the diameter can increase while keeping the stress within tolerable limits. The extrusion barrier can retract its original shape after the external force is removed. The structure can be forced radially outwardly on a ramp such as a cone and the resulting deformation is elastic. When the borehole wall or surrounding tubular is reached the leading coils conform to the wall shape and the coil gaps are closed. At the same time, a restoring potential energy force is stored in the set position so that on release of the packer the potential energy force acts to push the barrier back down the ramp while also urging the shape to retract radially inwardly to facilitate removal of the packer or plug. Those and other aspects of the present invention will be more readily apparent to those skilled in the art from a review of the detailed description of the preferred embodiment and the associated drawings while recognizing that the full scope of the invention is to be determined by the appended claims.